Method of producing corrosion resistant coatings on magnesium



Patented Nov. 11, 1941 UNITED STATES PATENT OFFICE METHOD OF PRODUCINGCORROSiON RE SISTANT COATINGS ON MAGNESIUM Robert W. Buzzard,Kensington, Md.

No Drawing. Application October 14, 1937, Serial No. 168,931

(Granted under the act 'of March 3, 1883, as

amended April 30, 1928; 370 0. G. 757) 4 Claims.

The invention relates to the treatment of corrosion resistant coatingson light metals and their alloys, especially magnesium, and one of theobjects is to increase the durability and the protective characteristicsof the coatings on the metal.

The method specifically relates to subjecting protective coatings onlight metals or alloys ,thereof to electrolytic treatment so as to forma seal that not only increases the durability of the metal to wear butalso improves its resistance to corrosion.

The metal to be treated according to this in vention is washed free ofthe primary coatingsolution, which may be applied by an electrolytictreatment or by the dip or immersion method, and is then subjected to anelectrolytic treatment in a solution containing one or more chemicalswhich attack the metal through the porous openings of the primarycoating, and fixes the coating in a more insoluble form thereby reducingthe porosity of the protective coating on the metal.

I have found that a very good and uniform seal for protective coatingson light metals can be obtained by anodizing the coated metal in asolution containing one or more substances selected from the groupscomprising chromium, borates, fluorides, sulphates, manganese and thecompounds of these substances. Either the sealing solution or the methodof application should difier from the solution or method used toform thecoating on the metal. I The concentrations of the substance orsubstances selected to make ;up the anodic sealing bath is limited onlyby the solubility of the substance and may vary from -1 gram per litersolution up to the saturation point of the particular substance. Thetemperature of the solution may vary from normal room temperature to 90C. and the current density may also vary from to 100 amperes per square.foot. The time of treatment depends upon the strength of the solution,the current density, and thequalityof seal desired. Good seals of coatedmetal have been obtained by subjecting the coated metal to. treatmentfor periods varying from 5 minutes to one hour.

Specifically, as an example, if an anodic coating be formed on magnesiumor its alloys in a bath of per cent dichromate and 2 per cent phosphateor if a dip, or immersion coating be formed on the metal in a bath ofsodium bichromate and nitric acid, and the resultant coating has beenwashed free of solution, its cormetal is then subjected to anodictreatment in a solution containing per cent dichromate.

As an example of a change in method of applying the protective coat andmy improved seal on metal, I formed a coating on two identical magnesiumspecimens by immersing the metal in a bath of 10 percent sodiumdichromate and 2 per cent monosodlum phosphate heated to approximatelyat or above the boiling point of water and then followed this treatmentby subjecting one of the coated specimens to anodic treatment in thesame bath. In tests the sealed specimen proved far superior to theunsealed,

specimen.

-'It should be understood that the protection obtained by my treatmentis dependent on the presence of a coating, either by an electrolytic ordip method, on the metal and is not a prime surface treatment.

As further examples of proportions of some of (g) 10 percent dichromateand 2.to percent phosphate. The coated metal may also be given two ormore sealing treatments, provided, the solutions in the coatingtreatment and.the subsequent sealing treatments difier from each other,however, at least one of the sealing treatments shall be anodicallyapplied iii-accordance with my invention herein described. As examples,magnesium coated in a bath of 10 percent dichromate and 2 percentphosphate may be first immersed in a solution of dichromate and chromiumfluoride followed by an electrolytic treatment in a solution of- 25percent by weight dichromate;

: or, coated magnesium may be first anodically rosion resistance willgreatly increased ifthe sealed in a solution of 5 percent boratefollowed by a second anodical sealing treatment in a solution of 25percent dichromate.

The above mentioned substances used in my improved sealing treatmentsare mentioned by way of illustration and not limitation, since theinvention comprehends treatments of the character indicated broadlyabove and is not to be limited except by the appended claims.

The invention described herein may be manu factured and used by or forthe Government oi the United States of America for governmental purposeswithout the payment of any royalties thereon or therefor.

What I claim is;

1. A method of producing corrosion resistant coatings on magnesium orits alloys, comprisingv forming a protective coating on the metal in anaqueous bath containing water soluble salts of dichromate, andphosphate, then immersing the coated metal in an aqueous solution ofdichromate and chrome fluoride: and then subjecting the treated andcoated metal to anodic treatment in an aqueous solution containing from20 percent to 75 percent by weight dichromate.

2. A method of producing corrosion resistant coatings on magnesium orits alloys, which comprises subjecting the metal to an anodictreatprimary coating on the metal by anodically treating in an aqueousbath containing at least one water-soluble salt selected from the groupconsisting, of the chromates, dichromates, fluorides, and borates 4. Amethod of producing corrosion resistant coatings on magnesium or itsalloys, which comprises forming a protective coating on .the metal mentin an aqueous bath containing a watersoluble dichromate and phosphate,and then in an aqueous bath containing a water-soluble dichromate andnitric acid, and then sealing the primary coating on the metal byanodically treating in an aqueous bath containing a watersolubledichromate.

ROBERT W. BUZZARD.

